In mobile networks, such as a 3GPP (Third Generation Partnership Project) mobile network, various types of terminal devices may be used. For example, Machine Type Communication (MTC) terminal devices, such as sensor devices, alarm devices, remote control devices, or the like may be used in addition to conventional types of terminal devices, such as mobile phones, smartphones, data modems, mobile computers, or other types of user equipment (UE). MTC terminal devices are typically characterized by a modest bit rate and sparse communication. MTC terminal devices may therefore be implemented with low-performance communication capabilities.
The existence of MTC terminal devices or other devices having similar data traffic characteristics may have an impact on the mobile network, in particular if the mobile network is designed to support large bandwidths or high throughputs, as for example in a mobile network according to 3GPP LTE (Long Term Evolution).
According to the Release 8/9 versions of the LTE specifications, the supported cell bandwidth is within the range of 6 and 100 resource blocks (RBs), about 1.4 to 20 MHz, and a UE is mandated to support all specified bandwidths in order to be standard compliant. Furthermore, UEs belonging to the lowest defined release 8/9 UE category need to support bit rates of at least 10 Mbit/s in the downlink and 5 Mbit/s in the uplink, which exceeds the needs of most MTC terminal devices. The ability to support high bandwidth, in particular bandwidth flexibility, and high throughput drives both cost and power consumption in the MTC terminal device. Accordingly, it is desirable to define low-performance UE categories in order to meet the characteristics of MTC terminals devices.
With the introduction of low-performance UE categories, it is possible to manufacture terminal devices with considerably less complexity and/or power consumption. In particular, the cost of each device is to a large extent driven by the silicon area. With significantly reduced throughput requirements, it is possible to reduce the size of hardware accelerators that are used to perform the most computationally complex operations, or even to replace them altogether with software implementations. Similarly, in a software-based implementation it is possible to replace large, possibly multiple, processors with smaller and/or fewer ones.
The potential to reduce the power consumption in a terminal device also strongly depends on the supported bitrates. Given a specific UE implementation, operation at the maximum supported bitrate requires a certain number of baseband processing operations, corresponding to a certain power consumption. Lower bitrates than the maximum one require fewer baseband processing operations, whether implemented in hardware or software. Consequently, support of lower bitrates may also be used for enabling reduction of energy consumption by the terminal device.
Even though it is possible to lower device complexity and/or power consumption when the throughput requirements are lower, there are typically still some timing constraints that need to be fulfilled in order to comply with a particular mobile network standard. For example, in 3GPP LTE one such timing constraint is associated with feedback messages of a Hybrid Automatic Repeat Request (HARQ) protocol which is used for reporting whether a transmission on the physical layer could be successfully decoded by the UE or not.
More specifically, in 3GPP LTE a synchronous HARQ feedback timing is used. For a downlink transmission this means that when a transmission of a transport block takes place the HARQ feedback message is sent by the UE after a fixed feedback time. Both, fixed timing as well as the specific resource allocation provides that no additional information is needed to map the HARQ feedback message to the corresponding terminal and the HARQ process it belongs to.
The resource allocation for the Physical Uplink Control Channel (PUCCH) is done via a predefined fixed mapping to the resources that are used on the Physical Downlink Control Channel (PDCCH) for sending a downlink assignment. The resources used on the PDCCH are called Control Channel Element (CCE). The first used CCE for the downlink assignment determines which PUCCH resource should be used for the HARQ feedback.
However, the above way of controlling HARQ feedback timing may be problematic when different types of terminal devices coexist in the mobile network, e.g., MTC terminal devices and conventional UEs. In particular, low-performance terminal devices may not be able to comply with the HARQ timing requirements in the same way as conventional UEs. Also, the HARQ timing requirements may be problematic for a UE which is in a low power mode.
Accordingly, there is a need for techniques which allow for efficiently handling feedback of a retransmission protocol used for transmissions of data blocks between a mobile network and a terminal device.